1. Field of the Invention
The present invention is broadly concerned with improved methods for startup and shutdown of preconditioners and complete food processing systems so as to avoid creation of substantial quantities of waste materials. Preferably, the invention is concerned with such methods, and optional flush methods, which employ software control of the system equipment in order to generate very little, if any, waste products. The invention is especially adapted for use with preconditioner/extruder systems used for the production of human foods and animal feeds.
2. Description of the Prior Art
Conventional food extrusion systems typically include a material feed system, a preconditioner, and an extruder. These are linked in serial order so as to deliver material to the preconditioner where it is moisturized and heated, and normally partially cooked, with the preconditioned material then directed to the extruder for final processing. Startup of such systems requires that the preconditioner and extruder achieve normal operating process conditions in terms of temperature, steam and/or water injection into the preconditioner and sometimes into the extruder, and pressure conditions within the extruder barrel. Normally, startup conditions are different than final processing conditions, and the former are adjusted over time to achieve the final process conditions.
In starting up a preconditioner, it is a common practice to initially divert the output of the preconditioner to waste, rather than passing this material into the extruder barrel. Similarly, when material is fed into the barrel, there is a period of time where unacceptable product is produced at the extrusion die, and this material is likewise considered waste. The average amount of waste material generated during startup of a preconditioner/extruder system depends upon the type of extruder (e.g., single or twin screw) and the process rate. Experience has shown that the waste ranges from about 80-170 kg.
This amount of waste is significant, especially in the production of short-run specialty products. However, the creation of such waste, especially at the preconditioner, can lead to unsanitary plant conditions. The under-processed waste material from the preconditioner is moist and warm, but has not been heated to a sterile condition. Therefore, this waste is subject to microbial contamination and growth, and must be carefully handled lest it serve as a source of contamination in the plant. These factors are to some extent mirrored with extruder waste, but generally the latter is not as troublesome as preconditioner waste.
Present day extrusion systems often are digital processor-controlled through a programmable logic controller (PLC) or other known device. In such systems, a formula including default or inputted values is created by the system operator, in order to control startup, normal running, and shutdown. U.S. Pat. No. 6,465,029 describes a software-based control system which is particularly designed for rapid changeover between extrusion runs, allowing the operator to easily change between formulas and final products without the need to shut down the system.
There is accordingly a real and unsatisfied need in the art for improved methods of system startup and shutdown without generating undue amounts of waste materials.